Add steps on proposing changes to Marquez

[wslulciuc]

Problem

Marquez does not currently outline steps for proposing changes or a template outlining a design doc for a proposal.

Solution

Add steps on how to submit a proposal for review wand a design doc tempalte.

Checklist

• You've signed-off your work
• Your changes are accompanied by tests (if relevant)
• Your change contains a small diff and is self-contained
• You've updated any relevant documentation (if relevant)
• You've updated the CHANGELOG.md with details about your change under the "Unreleased" section (if relevant, depending on the change, this may not be necessary)
• You've versioned your .sql database schema migration according to Flyway's naming convention (if relevant)
• You've included a header in any source code files (if relevant)

[codecov]

Codecov Report

Merging #2065 (bd61022) into main (1330831) will decrease coverage by 0.00%.
The diff coverage is n/a.

❗ Current head bd61022 differs from pull request most recent head 6a95c60. Consider uploading reports for the commit 6a95c60 to get more accurate results

@@             Coverage Diff              @@
##               main    #2065      +/-   ##
============================================
- Coverage     78.79%   78.79%   -0.01%     
  Complexity     1011     1011              
============================================
  Files           200      200              
  Lines          5574     5573       -1     
  Branches        422      422              
============================================
- Hits           4392     4391       -1     
  Misses          730      730              
  Partials        452      452              

Impacted Files	Coverage Δ
api/src/main/java/marquez/db/FlywayFactory.java	96.96% <0.00%> (-0.09%)	⬇️

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[mobuchowski]

Looks good to me